The present invention generally relates to an electric shaver and, more particularly, to an electric shaver incorporating a drive control for controlling the speed of rotation of a drive motor to an optimum value.
It is well known that, when an electric shaver is actually used for shaving beard, the optimum shaving is achieved if an electric motor used in the electric shaver is driven at about 6,600 rpm. This is illustrated in the graph shown in FIG. 1 of the accompanying drawings, which shows the relationship between the speed of rotation of the motor used in the electric shaver and the percentage of the number of beard bristles so cut as to represent a good cross-sectional contour relative to the total number of beard bristles having been shaved. The graph of FIG. 1 accounts that a higher or lower speed of rotation of the motor in the electric shaver than about 6,600 rpm would not ensure the optimum shaving performance. In practice, the actual speed of rotation of the drive motor effective to achieve the optimum shaving performance varies from 6,600 to 8,000 rpm depending on the type of a particular electric shaver.
In view of the foregoing, an electric shaver incorporating a drive control for controlling the motor at all times to a predetermined speed required to achieve the optimum shaving performance regardless of the magnitude of a load imposed on the motor has been proposed in, for example, Japanese Laid-open Patent Publication No. 61-62381, published Mar. 31, 1986, the invention of which has been assigned to the same assignee of the present invention. This prior art drive control is reproduced in FIG. 1 of the accompanying drawings and comprises a speed detector for detecting, and generating a speed signal indicative of, the speed of rotation of the motor, a digital-to-analog converter for converting the speed signal into an analog speed signal, a comparator for comparing the analog speed signal with a reference signal representative of the predetermined speed of rotation and for generating a difference signal indicative of the difference in level between the analog speed signal and the reference signal, and a control circuit for controlling the amount of voltage to be supplied to the motor in dependence on the difference signal thereby to control the speed of rotation of the motor to the predetermined value. The drive control is also shown as including a load detector for detecting a reduction in speed of rotation of the motor resulting from an excessive load imposed thereon and for increasing the voltage being applied to the motor in the event of the reduction of the motor speed below a predetermined value, and a display providing a visual indication of the occurrence of the reduction of the motor speed.
The above mentioned publication also discloses a specific wiring circuit of the drive control. Furthermore, the speed detector disclosed therein comprises a pick-up coil and an operational amplifier, said pick-up coil being disposed in the vicinity of the motor for detecting a change of the magnetic field generated by the motor during the rotation of the motor. The use of a light emitting element and a photoelectric sensor in combination with an interceptor for intercepting the path of travel between the light emitting element and the photoelectric sensor as a function of the motor speed is also suggested in the above mentioned publication as an alternative form of the speed detector.
However, the above mentioned publication is silent as to a specific arrangement and positioning of the photoelectric detector in relation to mechanical component parts of the electric shaver. As is well known to those skilled in the art, clippings of hair or beard gather in the vicinity of and around a reciprocable blade assembly cooperating with a stationary finely perforated shear foil. Moreover, not only may it not infrequently happen that the electric shaver receive shocks such as when slipped onto the floor or a sink, but also the electric shaver itself is subjected to vibrations. Therefore, care is required in selecting the arrangement and position of the photoelectric detector in consideration of the clippings and/or the vibrations.